Two dimensional pixel array light sensors may be used with a color filter array for achieving multi-color detection and imaging.
In addition, there are light sensor arrays that do not require color filters, as the detection of color is based on the depth of the light sensitive areas or pixels within a semiconductor substrate. These types of sensors may rely on the wavelength-dependent absorption coefficient of silicon (Si).
A double-layer photodiode has previously been described that is created in an integral structure to form two diodes, where an upper diode has a relatively thin active region and a lower diode a relatively thick active region. Light whose wavelength is to be measured is directed onto the upper diode. The thickness of the first diode is chosen so that, in the spectrum of light wavelengths being measured, the energy of the shortest wavelength is entirely absorbed in the first diode. As the radiation wavelength increases, the absorption by the upper diode decreases exponentially, and the unabsorbed light is transmitted through the thin active region into the thick active region of the lower diode where it is absorbed. The thickness of the active region of the lower diode is chosen so that it absorbs substantially all of the energy of the longest wavelength in the spectrum being measured. The absorption of the photon energy in each diode creates electron-hole pairs therein which produce in each diode a change in conductivity that is proportional to the absorbed energy. Since the difference in conductivities of the two diodes is a function of the wavelength of the incident light, as the wavelength changes, the difference between the changes in the conductivity of the two diodes, divided by the sum of the changes in the conductivity, is a function which is a single-valued function of the wavelength of the incident light, and which is independent of the intensity of the incident light. A measuring circuit connected to the double-layer diode provides a direct reading of the wavelength.
FIG. 1 shows the absorption coefficient of silicon (Si) as a function of wavelength.
A digital imager apparatus has been described that uses the differences in absorption length in silicon of light of different wavelengths for color separation. A preferred imaging array is said to be based upon a three-color pixel sensor using a triple-well structure. The array is said to result in elimination of color aliasing by measuring each of the three primary colors (RGB) in each pixel in the same location.
FIG. 2 shows a three-color pixel sensor that uses the triple-well structure.
FIG. 3 depicts a graph that plots light absorption length in Si versus wavelength.
A two dimensional 1-bit receptor array has been described as a digital-film sensor (DFS), which is an array of deep-SDL (sub-diffraction-limit) pixels, defined as those smaller than a 550 nm Airy disk diameter, wherein each pixel is a fraction of a micron in size. While several photoelectrons could contribute to pushing the output signal above some threshold, ultimately single photoelectron sensitivity is said to be desired. It is said that a pixel that only needs to detect a single photoelectron has much lower performance requirements for full-well capacity and dynamic range than an analog pixel in a conventional image sensor. These specialized pixels have been referred to as “jots”.
The implementation of a jot is said may be by making a conventional active pixel with very high conversion gain (low capacitance). Other approaches are said to include using avalanche or impact ionization effects to achieve in-pixel gain, as well as the possible application of quantum dots and other nanoelectronic devices. It is stated that stacked structures are also possible.
In operation, at the start of the exposure period the jot would be reset to a logical ‘0’. If it is then hit by a photon during exposure the jot is set to a logical ‘1’ immediately or upon readout. Due to the single-bit nature of the “analog-to-digital” conversion resolution, it is said that high row-readout rates can be achieved.
It is said that color can be handled in a manner analogous to current color image sensors. That is, the jots could be covered with color filters. In this case red (R), green (G) and blue (B) jots could be treated separately, and later the digitally developed images combined to form a conventional RGB image. R, G, and B jots need not appear at the same spatial frequency.